This invention relates to a vapor generating system and, more particularly, to such a system in which a vapor generator burns a relatively low BTU product as essentially free of sulfur which is generated by a gasifier located integrally with the vapor generator.
The Environmental Protection Agency and various state agencies have established standards of performance that define maximum allowable sulfur dioxide emission levels for fossil fueled power stations. In response to these standards, a generation of stack gas clean up equipment has been designed to remove, or scrub, sulfur dioxide from the steam generator flue gases prior to release into the atmosphere. Since large volumes of gas with dilute sulfur dioxide concentrations are encounted at the steam generator exit, the stack gas clean up equipment becomes large and expensive.
Instead of controlling sulfur dioxide emissions by treating the stack gases it is advantageous to remove sulfur from the fuel prior to combustion in the steam generator, since at this stage the volume of gases requiring treatment is significantly reduced. To this end a gasification process has evolved that involves the partial combustion of fuel, such as heavy fuel oil or particulate coal, in a fluidized bed of lime particles. Desulfurization is accomplished through reaction with the lime particles and a combustible off-gas is produced that is ducted to a steam generator where combustion is completed in commercially available gas burners.
However in these systems, hot gas ducting has to be provided along with a cyclone separator in the case of particulate coal, to pass the product gas from the gasifier to the steam generator. However, this equipment is expensive and, in addition, since the cyclone separators were less than completely efficient, the coal particles would enter the furnace and cause an appreciable build up of carbon. Therefore, the furnace and/or the gasifier had to be designed to burn the carbon, which often comprised the efficiency of the system. Accordingly, it is an object of the present invention to provide a two-stage combustion system with sulfur removal in the first stage gasifier and combustion of the gas together with the carbon particulates in the second stage furnace.